A major theme of this program project is the study of accessory cell function required for the stimulation of T helper cells. The two functions that are investigated are the expression and role of two key molecules: interleukin-1 and the class II histocompatibility molecules, or Ia molecules. Two of the projects deal with basic issues concerning some of the regulatory mechanisms that lead to IL-1 and Ia production by antigen presenting cells. Attempts are described to modulate Ia expression mainly by the use of by antibodies to interferon- gamma. The effects of this manipulation in the transplantation of allogeneic tissues is critically analyzed with a number of tissues. One of our objectives is to integrate the activity of basic immunobiologists with transplantation surgeons by creating a program where both activities are deeply interrelated for the benefit of both groups.